CHAPTER 7
Fabrication
Now that we have designed a PCB, the next step is to have the PCB made for us. Although it is possible to etch your own fairly crude PCBs, the cost involved is probably higher than having the boards made by an Internet service.
Internet services are also available that take the step beyond simply making the bare PCBs and include actually soldering the components onto it. This chapter will also tell you how to put your own components onto the PCB, for both through-hole designs and surface-mount projects.
Checking the Design
As my patient woodwork teacher at school would say in response to yet another pig’s ear of a dovetail joint, “Measure twice and cut once.” Nowhere is this truer than in PCB design. If you are using a service over the Internet, then unless you are paying for a fast turnaround, you likely will have to wait a couple of weeks for your PCBs to arrive. So the last thing you want is to get the PCB back and immediately notice something wrong with it.
Before you send off the design files to have your PCB made, check the design, using the techniques outlined below, until you think that it is perfect. It probably won’t be perfect, but it should work just fine and you should at least be close to making the next batch perfect.
Paper PCB
One of the quickest and easiest ways to start checking your design is simply to print a copy and then lay the components onto the paper in their correct positions. Before you print the PCB in Fritzing, select Both Layers from the Layers control and View from Above from the View control. The resulting printout will be the same size as the PCB. Figure 7-1 shows such a printout with some of the components laid on top of it.
FIGURE 7-1 A paper prototype.
Do the Components Fit?
The first thing to check, especially with a through-hole design, is that the parts you intend to use with the design will actually fit the pads of the PCB. There isn’t going to be a problem with the common components, unless you accidentally selected the wrong package when you were laying out the PCB. But it’s reassuring to check that everything will fit. You can even poke holes in the paper and push the component leads through, if you like.
There is more likely to be a problem with unusual components such as the AAA battery holder in the cycle lamp project, where the pin spacing is very specific to that particular part.
Is Everything Connected Correctly?
The second thing to check is that the design has all the necessary connections and no unnecessary connections. Since you can see the traces from both layers, a good technique is to also print the schematic diagram and then, for each net on the schematic, tick off the corresponding trace or fill on the PCB.
As you gain experience and confidence in Fritzing, you will probably check less and less as you get a feel for what might be risky, which will allow you to concentrate on likely problem areas.
PDF Files
The design files that you send away to the PCB service are called Gerber files. These files are images in a special format that can only be viewed with a special viewer (see the next section). However, you can have Fritzing produce an equivalent set of PDF files that can be opened just as any other PDF. Checking the PDFs should be good enough, because unless there are bugs in Fritzing’s Gerber file generation code, if the PDFs are correct, the Gerber files will be correct.
To generate a set of PDF files, select the menu option Export | For Production | Etchable PDF. This will open a file dialog where you will select a directory to which all the PDF files will be written. Create a new empty directory for this to make it easier to find the files.
These PDFs can also be used for do-it-yourself (DIY) PCB production using photoetching, milling machines, or toner transfer.
Figure 7-2 shows the folder containing the generated PDFs for the cycle lamp project.
FIGURE 7-2 Generated PDFs for the cycle lamp project.
There are actually 16 files, but you really don’t need to check all of them. The most critical part of the design is that the copper traces are correct. So the first files to check are the top and bottom copper layers. Figure 7-3 shows the two files copper_bottom and copper_top.
FIGURE 7-3 The bottom and top copper layers.
You can open these in Adobe Reader or any other PDF viewer. Printing them on thin paper will allow you to superimpose the layers by holding them up to the light. But beware; copper fills will soon eat through your printer’s ink supply.
Alongside the PDF files for the view from the top of the board, there is also a parallel set of image files labeled “Mirror,” which will be the view from the underside of the board.
In addition to the images for the copper layers, you will find the following image files.
mask_top and mask_bottom
The black areas on these files show where the copper layer will be exposed. The white areas will be covered in insulating solder mask.
paste_mask_top and paste_mask_bottom
These files are only used in surface-mount designs and then only if the design is going to have surface-mount components attached during manufacture, or you are buying a solder paste stencil (see the section on PCB fabrication for surface-mount technology later in this chapter). Figure 7-4 shows the top layer of this file for the surface-mount version of the cycle lamp project.
FIGURE 7-4 The top paste mask layer.
As you can see, the black areas correspond to the surface-mount pads. During the process of attaching the SMDs, a stencil will be created with the black areas cut out of a metal or mica sheet. They are then placed over the newly made PCB, and solder paste is smeared over the stencil with an applicator so that when the stencil is removed, all the pads are covered with a layer of solder paste.
silk_top and silk_bottom
The silk layers will contain the text and other markings that will appear on the PCB’s solder mask layer.
Gerber Viewers
The techniques described above should give you sufficient reassurance that your design is just fine. If you need more, then you might like to view the Gerber files that will eventually be used to manufacture the PCB. To do this, you will need a special viewer such as MCN Gerber View for the Mac (www.mcn-audio.com/sharewares/) or you can use an online viewer such as www.gerber-viewer.com/.
You will need to create the files in much the same way as you exported the PDF files, only this time, select the option Extended Gerber (RS274X).
Figure 7-5 shows the MCN Gerber viewer in action. Very usefully, you can toggle the visibility of each of the files, allowing you to superimpose them on top of each other.
FIGURE 7-5 Use of a Gerber viewer.
One layer missing from the PDF files is the drill file that shows where the holes are. This is present in the Gerber files and has the extension .TXT.
PCB Fabrication
Now that you are happy with your design, it is time to actually make the design for real. Although you can make your own PCBs using photoetching, toner transfer, or milling machines, frankly the results will look amateurish. There is an element of craft and trial and error in all these methods, when you do them at home. So if you enjoy that kind of thing, take a look at some of the many online tutorials that cover these processes.
An interesting development is the arrival of machines such as the Voltera V-One machine (http://volterainc.com/) designed for amateur use to create PCBs on your desk as easily as 3D printing. Until such machines are cheap and readily available, you will probably be using a PCB service.
Choosing a Service
There are lots of PCB services out there, not the least being Fritzing itself. Hovering over the Fabricate button in Fritzing will even tell you how much your PCB will cost. You also have the reassurance that if your design is made using Fritzing, there should be no problem at all with design rules that you might need to check if you used another service. Using Fritzing will also help support the continuing development of this excellent free software.
The process of having your PCBs made will be much the same whether you use Fritzing or some other service. The example I use later is for the service offered by Seeed Studio, but there are new providers cropping up all the time and it can pay to shop around. You will find some providers listed in the Appendix.
Most providers will expect you to upload a zip file containing the Gerber files or sometimes to e-mail the file to them. Conveniently, most will then use this information to give you a price for the fabrication. Another advantage of using Fritzing is that you can just upload your .fzz file and they will generate the Gerber files.
Fabrication with Fritzing.org
If you plan to use the Fritzing service, you can just click on the Fabricate button at the bottom of the Fritzing window when you are on the PCB tab. Clicking on the Fabricate button will open the page http://fab.fritzing.org/fritzing-fab in your browser, where you can start the process of ordering your boards.
Click on the Submit Your Order button. This will prompt you to log in or register with Fritzing. Once you have done this and clicked on the e-mail confirmation link that will be sent, you can click on the Add Sketch button on the Web page, navigate to the .fzz file for your design, and say how many copies of the board you want.
At this point, you will be told the cost and you can go ahead and order the boards.
Fabrication with Other Services
Fritzing offers a premium service and will do a certain amount of manual checking of your design before they go ahead and make the PCBs. They are not going to check your electronic skills, but will probably spot any glaring problems with your project. There are, of course, lower-cost services that will make your boards exactly as you specify without much manual checking. Such services will not work directly from your .fzz file; instead you will need to export the Gerber design files as described earlier.
Gerber Files for Fabrication
You may have already generated your Gerber files, if you decided to view them in a Gerber viewer. If you didn’t export them earlier, then export the files now. The list of files that will be generated by Fritzing is summarized in Table 7-1.
TABLE 7-1 List of Files Generated by Fritzing
Note that if your design is entirely through-hole, no pasteMask files will be generated.
Fabrication Guidelines
Each fabrication service will have its own guidelines about what you can and can’t do in your PCB design. The Fritzing software uses sensible defaults for everything and produces Gerber files that are very unlikely to cause problems for any fabrication service. But, just in case, let’s look at a typical set of guidelines provided by Seeed Studio for their Fusion PCB Service.
The service starts by explaining what files it will need to produce the bare PCB without any assembly. It is common for just the file extension to be used to identify the file, even though Fritzing includes a more descriptive name for each file. The descriptive name will not be a problem, but you must make sure to supply a set of files with the extensions expected. The list below is taken from Seeed Studio’s PCB Order Submission Guidelines page.
Top Layer: pcbname.GTL
Top Solder Mask: pcbname.GTS
Top Silkscreen: pcbname.GTO
Bottom Layer: pcbname.GBL
Bottom Solder Mask: pcbname.GBS
Bottom silkscreen: pcbname.GBO
Board Outline: pcbname.GML/GKO
Drills: pcbname.TXT
Most of the file names coming out of Fritzing are just fine for this, and the three extension letters can be uppercase or lowercase. The only potential problem with the set of files from Fritzing is that it includes two .txt files: drill.txt and pnp.txt. The specification above specifies that the drill file should have an extension of .TXT, so delete the file pnp.txt from the files generated by Fritzing, to avoid any possible confusion. The pnp (pick and place) file is used only if the PCB is going to be populated with components.
Make sure that your folder contains a file with each of the extensions listed above and then zip the folder. This is the folder that you will upload to the service.
You should also find a long list of specifications for the PCB in the submission guidelines. This is quite long for the Seeed Studio service, so I will just pick out some of the key things to look for in Table 7-2.
TABLE 7-2 A Sample of Board Specifications
A minimum trace width of 6 mils is very fine; the smallest size that Fritzing provides in its drop-down list is 8 mils. The specification also specifies a minimum text height. This is unlikely to be problematic as it is almost microscopic. Note the relatively small upper size limit for drills of 6.35mm.
Fabrication with Seeed Studio
Once you have ensured that your design is not contravening any of the fabricator’s rules, it’s probably time to upload the Gerber files and choose any options for your PCB fabrication. Here the process is usually similar to using the Fritzing service except that you usually upload a zip archive file containing the Gerber files. Some services ask you to e-mail the files rather than upload them.
Once you have uploaded the design files as a single zip archive, the service may provide some automatic checks on the files and calculate the PCB size; or you may have to set that option manually, as is the case with Seeed Studio. Figure 7-6 shows the Seeed Studio options page, with the PCB dimension set to “5cm Max*10cm Max.” The size specified here needs to be large enough to completely enclose the PCB, whatever shape it is.
FIGURE 7-6 Setting options in Seeed Studio.
Most of the options can be left at their default values.
• The default for PCB thickness is 1.6mm, and this is fine for most situations. If you have a very small or large PCB, then you may want to decrease or increase the thickness, respectively.
• The quantity defaults to 10 boards, but 5 boards is an option that saves a little money and will be more than enough for a prototype. All services including Fritzing have much lower cost per board, the more boards you order.
• Most services will offer you a choice of different colors for a little extra cost.
• The copper weight option is usually 1oz per square foot of copper, but you may get an option of 2oz for extra cost.
You may also see an option for panelizing. Panelizing means repeating the same PCB design on the same area of PCB material, with a groove scored almost all the way through, between each PCB, so that they can be snapped apart. The idea is that if the PCBs have a standard size of 5cm × 5cm, you could fit 10 boards 1cm × 2.5cm onto the same square, saving you a lot of money.
Some fabrication services just ban panelizing outright, and others will let you do it for an extra cost. However, Fritzing does not generate Gerber files to support panelizing, so it’s a moot point.
The rest of the ordering process is just like any online retailing. You hand over your money and sometime in the future your PCBs will arrive, ready for you to assemble them, but first you are going to need some parts.
Parts and BOMs
BOM stands for Bill of Materials. This is basically a list of the parts you use in your project. Eventually, when it comes to having your design manufactured, the BOM will probably be the most significant factor in determining the production cost of your design.
As well as exporting the Gerber files and PDFs, Fritzing can export the BOM for you as an HTML page (Figure 7-7).
FIGURE 7-7 Exporting a BOM from Fritzing.
You will find the option to export the BOM under the File | Export menu.
The file generated will tell you all the parts you need. But if this is eventually going to become a product, you will probably want to paste its contents into a spreadsheet, so that you can add some columns indicating sources for the various components as well as their costs.
Finding suppliers for the parts can be very time-consuming. The big international suppliers such as Farnell, Mouser, and Digikey stock so many thousands of components that it can be hard to see the wood for the trees. However, they do allow you to get your prototyping components from one place. In fact, bulk discounts for high quantities will generally make the large suppliers quite competitive in price, especially for very low-cost components such as resistors and capacitors.
Later, when it comes to finding the cheapest source for each component, the website http://octopart.com is invaluable for finding suppliers of components. Octopart is effectively a search engine for parts suppliers. You just enter the part number that you are looking for, and the price at all the suppliers that stock it will be displayed.
Now that you have a bare PCB and a load of parts, all that remains is to solder one to the other to make your prototype.
Soldering
Hand soldering is a skill that is easy to do, but difficult to do really well unless you have a talent for it. It is, however, very satisfying to solder the parts for your project onto a fresh new PCB.
Prototyping Services
If you really don’t want to get into soldering and invest in soldering equipment, then some PCB fabrication services will also do the soldering for you for a fee. In addition to the design files, they will want you to complete a BOM spreadsheet that includes sources for every component.
Tools
You do not have to spend a lot of money on tools for soldering PCBs. You can get perfectly good results with low-cost equipment. You wouldn’t learn the violin on a Stradivarius, so don’t get a top-end Weller or Hakko soldering station as your first soldering iron. Gradually improving your tools is one of the joys of electronic construction. Where would the fun be if you had the best of everything from day 1?
However you plan to do your own construction, there will be certain tools that you will need when soldering a PCB.
Snips and Pliers
Snips are used to cut the excess lead off components after they have been soldered. They are sharp and allow you to get close to the solder joint. They are also useful for stripping the insulation off wire. Eventually snips lose their sharpness and become blunt, especially if they are abused by being used to cut steel guitar strings. I use very cheap snips (see Figure 7-8) and then replace them as soon as they become blunt enough to be irritating to use.
FIGURE 7-8 Snips and pliers.
Long-nosed pliers last longer and are generally a useful tool to have around. They can be used to grip insulation tightly while you strip the insulation off a wire, or for holding onto components that you are trying to desolder from a board without burning your fingers. You might find yourself desoldering a board because you may have put the wrong-value resistor in place, or you may want to swap out a component to improve the design.
Multimeter
In a perfect world, everything would work the first time that you powered it up. The reality is that life is not quite like that. A multimeter (Figure 7-9) is an essential tool that will allow you to diagnose problems with your designs.
FIGURE 7-9 A multimeter.
You do not need to spend a lot of money on a multimeter. A basic entry-level multimeter costing just a few dollars will do just fine most of the time. The most important setting that you will use most of the time is DC volts on a range of 0 to 20V.
It is also useful to have a DC current setting of up to a few hundred milliamperes and a continuity test that buzzes when the test leads are connected. Everything else is just bells and whistles that you might use once in a blue moon.
Most of the time, accuracy is pretty irrelevant too. When things go wrong, it is usually a matter of orders of magnitude. So if your multimeter indicates a current of 10mA when the current is actually 12mA, usually that is good enough. It’s when the current is 100mA and you were expecting 10mA that there is a problem.
Soldering Station
While you can get by with a soldering iron that plugs directly into an AC outlet and has no way of adjusting the temperature, it is worth spending a few extra dollars on something that is thermostatically controlled and can accept fine-point tips (Figure 7-10). Make sure that you avoid anything advertised as being suitable for plumbing use.
FIGURE 7-10 A low-cost soldering station.
When you are buying your soldering station, bear in mind that eventually the tips (also called bits) will need to be replaced. So make sure that replacements will remain available, or buy them when you buy the iron. With the trend for components to get smaller and smaller, you will probably want a tip of perhaps 2mm. There are many different shapes of tip, and it is a matter of personal preference. Many people prefer a chisel-shaped tip. A simple conical tip is another popular choice.
If you plan to use lead-free solder, then temperature control is a must. You can get away with a simple low-cost soldering iron if you are using solder with lead in it, as this is just much easier to work with (see the next section).
WARNING It should go without saying that soldering irons get hot enough to burn your skin. So be very careful and always put the soldering iron back into its holder, as soon as you have finished with it. Do not leave it on the desk to roll off, triggering the automatic instinct to try and catch it, when inevitably its lead gets snagged and it falls off the desk.
Soldering also produces fumes from the rosin flux. It is a good idea to solder next to an open window, or use a fume extractor.
Solder
Traditionally, solder (Figure 7-11) has been made from tin and lead. Usually, this is in the proportion of 60 percent tin and 40 percent lead. The solder looks like a solid metal wire, but actually will normally have a core of flux rosin that helps the lead to flow when it melts. Legislation on the use of toxic chemicals has caused a reduction in the use of lead-based solder in favor of lead-free solder.
FIGURE 7-11 A reel of 0.7mm tin, lead solder.
This type of solder is an alloy of tin, silver, copper, and small amounts of other metals. It looks like lead solder and still has a rosin core, but is more brittle than lead solder and has a melting temperature of about 200°C (392°F) versus about 190°C (374°F) for leaded solder.
The differences do not end there. Many people find lead-free solder much harder to work with. It does not flow as easily as lead solder.
Lead solder is still widely available, and unless you are producing a product that you are going to sell, then it is really a matter of personal preference which type of solder you use. I know electronics enthusiasts who have a roll of lead-free solder that they use most of the time and then a roll of “the good stuff” (lead solder) when they have something tricky to solder.
Whatever type of solder you use, you will have another choice to make of what gauge of solder to buy. Two popular sizes are 0.7mm and 1.2mm diameter. Use 0.7mm or similar, as it is much easier to use when there are IC leads close together. If you need to solder some large terminal, you will find yourself feeding in quite a length of the narrow solder to deliver the required amount, but this is not really a problem.
Desoldering Braid
Desoldering braid (Figure 7-12) is not an essential tool for soldering; however, it can come in very handy from time to time. As well as its primary use for “unsoldering” components, it is also great for mopping up excess solder, especially when hand-soldering surface-mount devices.
FIGURE 7-12 Desoldering braid.
The braid is made of copper impregnated with flux that encourages the solder to flow. So when you place it between the pad that you want to remove the solder from and the soldering iron tip, it soaks up the solder as a sponge would. Once this is done, that section of braid cannot be reused; you snip it off and throw it away.
Tip Cleaner
When you solder, it is very important that the tip of the iron be clean, or you will end up with blobs of solder that do not make a good joint. There are two methods of cleaning the tip, both used with the soldering iron hot. One is to use a damp sponge, and many soldering stations include a sponge holder. The other is to use a container of brass wool, rather like a scouring pad (Figure 7-13).
FIGURE 7-13 Brass soldering iron tip cleaner.
The only real advantage of using a damp sponge is that it makes a great hissing noise as the hot tip of the iron is rubbed across it. It does, however, suffer from a number of disadvantages:
• The thermal shock of cooling the tip quickly as it comes into contact with the wet sponge will shorten the life of the tip.
• You have to keep wetting the sponge and need a supply of water.
Tools for Surface-Mount Devices
When attempting surface-mount soldering by hand, you will probably need all the equipment that we have just described for through-hole soldering. In fact, you can get away with just using regular soldering equipment if you select the larger SMDs. However, there are a number of special items that make surface-mount soldering simpler.
Hot Air Gun
A hot air gun (Figure 7-14) has interchangeable nozzles of different sizes and allows you to deliver a stream of hot air to an area of a circuit board.
FIGURE 7-14 A hot air gun.
You can normally set both the temperature of this air and the flow rate. You will need to control the flow rate, because many SMT components are so small that they can easily be blown away by the pressure of air from a hot air gun.
Solder Paste
When soldering surface-mount devices, with care you can use regular solder. However, if you are using a hot air gun or a reflow oven, then you will need to use solder paste (Figure 7-15). Solder paste is available in both lead-based and lead-free varieties and suffers the same pros and cons as those variants of regular solder.
FIGURE 7-15 A syringe of solder paste.
Solder paste is made from microscopic spheres of solder in a suspension of flux. For industrial use it is supplied in tubs; for small-scale use, you can buy it in syringes ready for hand use. Solder paste should be kept in your fridge but warmed up to room temperature when you are ready to use it.
WARNING Solder paste is a liquid. If you are using lead-based solder paste, it will easily find its way into the pores of your skin if you get it on your fingers. So, always wear latex gloves, if, like me, you are a bit messy and are likely to get it on your fingers.
Tweezers
To be able to pick up and place SMDs onto a board, you will need tweezers (Figure 7-16).
FIGURE 7-16 Nonmagnetic tweezers.
The tweezers should have a fine point and most importantly be nonmagnetic. If they are even slightly magnetic, then SMDs will probably stick to them, as many contain ferrous metals.
Magnifier
It can be really hard to see what you are doing when you are working with SMD. A large magnifying work lamp, such as that shown in Figure 7-17, can be a great help.
FIGURE 7-17 A magnifying work lamp.
These devices have a lighting ring around the lens that evenly illuminates the board you are working on. Since you are looking through the lens with both eyes, all-important depth perception is preserved.
Some people take this a stage further and use a binocular microscope. These are available specifically for working on circuit boards, and a zoom version will allow you both to work on boards and inspect them very closely for any problems. You should look for something that magnifies between 5 and 20 times.
Reflow Oven
When you are developing single boards, hand soldering works okay. It is a little tedious and time-consuming, but can be done. The professional way to attach SMD components to a board is to use a reflow oven.
The basic idea is that you put solder paste on the pads of the board, place the components onto the pads, and then bake the entire board in an oven to melt the solder paste and attach the components. We will see how to do this in a later section.
Commercial reflow ovens are quite expensive, but many people make their own by using low-cost toaster ovens, such as the modified device of the author’s shown in Figure 7-18.
FIGURE 7-18 A modified toaster oven.
WARNING These types of toaster ovens are often called “fire starters” and for good reason. They are very simple designs, with little in the way of thermal insulation. That means that they get very hot, and if you modify them, they can become even more dangerous.
If you decide to make one of these, never leave it unattended or anywhere near anything that could burn.
The model shown in Figure 7-18 has been modified to replace the thermostat with a proportional power control module, and a digital thermometer has been added to allow the necessary accurate monitoring and control of temperature.
Through-Hole Soldering
Having explored the various tools we will need, we start by learning how to solder through-hole PCBs. It is a good idea to try to follow these instructions on a PCB. You may wish to order one of the PCB designs from earlier in the book, such as the cycle lamp example.
Through-Hole Soldering Step by Step
First turn on your soldering iron and set the temperature. You will find conflicting advice for temperatures to use, but I set my soldering iron to 280°C (536°F) for lead-based solder and 310°C (590°F) for lead-free solder. Once you get comfortable with soldering, you will probably want to work at a higher temperature where the solder melts a bit more quickly. The higher temperature will not damage the components as long as you work quickly.
When the soldering iron is up to temperature, clean it on the damp sponge or brass tip cleaner. Once cleaned, it should look bright and silvery.
The key to soldering is not to heat the solder, but rather to use the soldering iron to heat the place where you want to solder and then feed solder onto that junction so that it melts and flows over the pad and the component lead.
First push the component leads through their holes and turn the PCB on its back (Figure 7-19a); then hold the tip of the soldering iron to the junction of the pad and the lead. Next, feed solder into the joint so that it flows all around the lead and covers the pad (Figure 7-19b). Once it has flowed all around, you can stop adding solder and move the tip away. You do not want the pad to be heaped high with solder. The solder should ideally form a nice mountain shape around the lead (Figure 7-19c). Finally, you can snip off the excess lead (Figures 7-19d and e).
FIGURE 7-19 Soldering a resistor.
When soldering components onto a PCB, you can make life much easier for yourself if you start with the components that lie closest to the surface of the board. That way, when you turn the board on its back, the weight of the board will keep the components pressed against the board.
Figure 7-20 shows the PCB for the cycle lamp project with all the components except the battery holders attached.
FIGURE 7-20 The cycle lamp example soldered.
If you make a mistake and find yourself needing to desolder a joint, then Figure 7-21 shows the steps you should take.
FIGURE 7-21 Desoldering.
If the board has not just been soldered, then you might find that it is quite hard. So heating it and adding a bit of solder could actually make it easier to desolder the component. After optionally resoldering the joint as described above, you place an unused end of the soldering braid against the joint (Figure 7-21a) and then press it down onto the solder pad, using the tip of the soldering iron (Figure 7-21b). As the solder melts, it should be drawn into the solder braid. You will be lucky to draw off all the solder in one go, so most likely you will need to snip off the now solder-covered end of the braid and repeat the process of pressing it against the joint. Eventually, you should have most of the solder removed, and the joint will look like Figure 7-21c.
Repeat this for the other lead or leads of the component. If you are very lucky, you will just be able to wait until the component has cooled and then gently pull the component back through the hole from the top. However, it is more likely that there will still be a little solder holding the component in place. If this is the case, then hold one component lead from the top side with long-nosed pliers and heat the pad from underneath while exerting a gentle pull on the lead to pull it back through. If you cannot get to the lead and you don’t mind breaking the component, then snip it in half, or snip the lead off on the top to make it easier to pull through.
All this means that you are likely to be heating the board for a while, which can make it look scruffy or even damage the board. Pulling the lead through with force is also likely to damage the PCB, and ultimately too much heat will eventually cause the pad to separate from the board.
SMD Hand Soldering
As long as the SMDs you use are at the larger end of the size scale and have pins on the edges of the devices rather than on the underside, they can be soldered relatively easily with a regular soldering iron. The main problem is that they are so light and since they do not have leads projecting through holes, there is nothing to hold them in place when you try to solder them.
Soldering Two- and Three-Leg Components
The sequence of steps to solder a 1206 resistor into place is shown in Figure 7-22.
FIGURE 7-22 Using a soldering iron on an SMD resistor.
This is one of those situations that would really benefit from having three hands—one to hold the soldering iron, one to hold tweezers to keep the SMD in place, and one to apply the solder. If you do not have three hands, then a good trick is to place a small mound of solder on one of the pads (Figure 7-22a) and then, while holding the SMD in place with the tip of your tweezers, press its lead into the little solder mound (Figure 7-22b) with the tip of your iron. The SMD will now stay in place without the need of tweezers as you solder the other end normally (Figure 7-22c). It’s then usually a good idea just to touch the first end with the iron and a little solder, just to freshen it up.
An alternative technique is to place solder paste onto the pads, hold the SMD in place with the tip of your tweezers, and then touch the tip of the iron to each lead until the solder paste beneath melts.
Soldering IC Packages
The approach above works just fine for two- and three-leg devices, but when it comes to ICs, this can be trickier. You can try the conventional soldering iron approach above, pinning the IC down with one corner pin and then carefully soldering the remainder of the leads, but often the solder will make unwanted bridges between the pins.
A good trick is not to worry too much about these bridges, but when you have finished soldering, lay desoldering braid along the row of pins and heat along the whole length to remove the excess solder. This is shown in Figure 7-23. This will still leave enough solder under the IC pins to make good connections.
FIGURE 7-23 Hand soldering an SMD IC.
SMD with Hot Air Gun
Generally speaking, it is much easier to solder SMD devices with solder paste and a hot air gun than with a soldering iron.
Soldering Two- and Three-Legged Components
The steps for soldering with solder paste and a hot air gun are illustrated in Figure 7-24.
FIGURE 7-24 Soldering with hot air gun.
First, place a small amount of solder paste on each pad. You can squeeze it out through the syringe needle. Use clean gooey paste, wiping away any crusty paste from the end of the syringe before you start (Figure 7-24a). Occasionally, I find a wooden toothpick useful for spreading the paste around a bit. Do not worry if it is a little untidy; when it melts, surface tension will cause it to pull back onto the solder pad.
Next, using your tweezers, place the component onto the pads (Figure 7-24b). Now fit a small nozzle onto your hot air gun, set the temperature to 280°C (536°F) for lead-based solder and 310°C (590°F) for lead-free solder, and set the flow rate to perhaps one-quarter of full power. When the air gun is up to temperature, pin the component down with the tip of your tweezers and then play the air gun over the component and its leads until the solder paste melts (Figure 7-24c). When the solder melts, you will see it change from dull gray to shiny silver and see it spread across the pad. While still pinning down the component, put the hot air gun safely back on its stand; after a second or two, when you are sure the solder has set, let go with the tweezers.
If you do not hold the component in place with tweezers, then even at very low airflows the hot air gun will probably blow the component out of position.
Soldering IC Packages
Soldering ICs is very similar to the process just described. The only real difference is that you may well want to clean up the connections using soldering braid, as shown in Figure 7-23.
Packages with Hidden Connections
Some IC packages have inaccessible components on their underside. To solder these, put paste on the pads as usual, and then while you hold the IC in place with the tip of your tweezers, place the hot air gun over the whole IC, until you feel that the solder has melted. If you do this for too long, you may damage the chip.
Using a Reflow Oven
By far the quickest way to solder an SMD board of any complexity is to use a reflow oven. It has the advantage that you only have to place the components on top of the solder-pasted pads. Once that is done, the whole board is cooked in the reflow oven, soldering all the components in one go. What is more, once you gain confidence, you can cook a whole batch of PCBs in one go. In any case, the “cooking” process only takes a couple of minutes.
If you have a board that contains both surface-mount and through-hole components, then solder the surface-mount components first.
Get Everything Together
Solder paste will dry out after maybe half an hour, so before you do anything, make sure that you have all the components that you need and that you know exactly how they will fit onto the PCB (which way around LEDs, etc., are to be placed). I find it useful to actually put the components onto the PCB without any paste just to make sure that I have everything I need. I have the board on a paper printout of the PCB, so that when I am sure I have everything in place, I can move everything off to one side, keeping the same relative positions of the components.
Another approach is to use labeled bottlecaps or even just circles on a sheet of paper labeled with the component’s part and/or value. Just don’t sneeze on your SMDs or they will be scattered far and wide.
Applying Solder Paste
The low-tech way of applying solder paste is the same as we described earlier when we looked at using a hot air gun. Simply go around the board, adding a little blob of solder paste onto every pad on the PCB by using the syringe dispenser. This is the time-consuming part.
The alternative to using a syringe is to use a stencil. Many PCB manufacturing services (for a small extra fee) will also supply you with a stencil. This can be made of thin steel, Mylar, or other materials, and it is placed over the PCB. It masks out most of the PCB surface except for the areas where solder paste needs to be deposited. You then place some solder paste on the stencil and “squeegee” the solder paste into all the “holes” in the mask. The excess solder paste is then scraped up and the template removed, leaving solder paste on all the pads.
You can also make your own stencils. If you search the Internet, you will find various DIY techniques for doing this by using laser cutters or vinyl cutters or even transferring toner onto a cut-up aluminum soft drink can that is then put in acid to dissolve away the holes and make the template.
In this example, we are applying the solder paste by hand to the fairly densely packed PCB that has a wide variety of different component types. When the solder paste has been applied, the board will look something like Figure 7-25.
FIGURE 7-25 The board prepared with solder paste.
Note what a poor job the author has done in supplying the solder paste evenly. You should aim to be neater than this, but even with this level of messiness, it is still likely to work.
Populating the Board
Starting with the smallest, lowest-lying components at the far end of the board, place them on the pads, using tweezers, until all the components are placed (Figure 7-26).
FIGURE 7-26 The populated board.
The board is now ready for cooking.
Baking the Boards
If you have a proper reflow oven, then baking the boards is pretty much as simple as putting them in and pressing a button. Behind the scenes, there is some fairly careful temperature control going on.
Solder paste requires a certain profile of changing temperature over time for it to do a good job of soldering components in a reflow oven. It has to go through four distinct stages:
• Preheat: Activate the flux.
• Soak: Warm the whole board to just below the solder melting point.
• Spike: Do this as fast as you can above the melting point to reflow the board.
• Cool: Cool everything before the components and board are damaged.
Each of these stages has precise temperatures and timings associated with it. A commercial reflow oven will allow you to select from preset profiles to match the paste you are using and control all aspects of the heating. Figure 7-27 shows the temperature profile of some leaded solder paste, and Figure 7-28 shows the board in the author’s homemade oven.
FIGURE 7-27 Leaded solder paste reflow profile.
FIGURE 7-28 Cooking the board.
Using a homemade device like this will never be as reliable as using a professional oven, but it is fine for prototyping. Making one of these is dangerous and should be undertaken only if you really know what you are doing. You can find instructions for doing this at the following Web pages:
• www.sparkfun.com/tutorials/60
• www.freetronics.com/pages/surface-mount-soldering-with-a-toaster-oven#.Us_cUGRdVyF
Both of these tutorials describe how to manage the temperature by hand, without the need for a complex controller. The final board is shown in Figure 7-29. You can see that the LED has moved a little. This is an effect of my cavalier application of solder paste, but can easily be corrected with a little hand soldering.
FIGURE 7-29 The final board.
Before you power up any board that you have made, you need to go over the whole thing very carefully with a magnifying glass, to check that there are no accidental solder bridges and that all the pins are soldered to pads. Be especially careful around SMD ICs. You can mop up excess solder causing bridges by using desoldering braid.
Depending on the solder paste that you used, you may also find little patches of flux and even tiny balls of solder on the board. I have a soft toothbrush that I use just to brush over the board. This will also highlight any loose components by brushing them off the board. This can be improved if you look for “no-clean” solder paste.
Summary
In this chapter, we have learned how to get a PCB made for us and then to solder the parts onto it. So now you should be familiar with the process of creating a fully populated PCB. In Chapter 8, we will look at ways of using Fritzing with the Arduino and other popular microcontroller platforms.